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Genetics of cleft chin
Summary
Some people have a prominent dimple or crease in the front of the chin, called a cleft chin (or "butt chin"); others do not. This is sometimes said to be controlled by a single gene with two alleles, with cleft chin (C) dominant to smooth chin (S). The small amount of genetic data that is available shows that this myth is not true; in addition, there are many chins that are intermediate between clearly cleft and smooth.
Are there just two kinds of chins?
Cleft chins come in a variety of shapes, including vertical furrows, Y-shaped furrows, and round dimples (Günther 1939). They also vary in depth from barely noticable to extremely prominent (Bhanu and Malhotra 1972). A few years ago, I looked through a gallery of United States Senators (because they have large official portraits online) and picked out the ones that had some hint of a cleft chin, along with a few smooth-chinned senators for comparison. As you can see, there are a lot of people with vague depressions in the middle of the chin, and few with really unambiguous furrows or dimples. I find it hard to draw a clear boundary between cleft and smooth chins in these pictures, so I think that dividing chins into two distinct categories, "cleft" and "smooth," is arbitrary and subjective.
The reported frequency of cleft chin varies widely among different populations; Indian populations range from 4 to 71 percent cleft chin (Bhanu and Malhotra (1972). Günther (1939) recorded cleft chins in 9.6 percent of German men and 4.5 percent of German women, while Beckman et al. (1960) said that 52% of boys and 26% of girls in Sweden have a cleft chin. Since all of these studies subjectively divided chins into cleft vs. smooth, it's not clear how much of the variation among studies is real differences among people, and how much is different judgements about which chins are cleft chins.
Family studies
Lebow and Sawin (1941) first suggested that cleft chin was a genetic character, based on data from a single family. They suggested that cleft chin was recessive, although they admitted that they didn't have definitive evidence.
There is an unpublished dissertation by Pfannenstiel (1951), which I haven't seen, that concludes that cleft chin is dominant, but affected by the environment (cited in Beckman et al. [1960]). Beckman et al. (1960) report the following data from Swedish families:
| Parents | C offspring | S offspring | Percent C |
|---|---|---|---|
| C x C | 10 | 1 | 91% |
| C x S | 24 | 28 | 46% |
| S x S | 4 | 33 | 11% |
If cleft chin was caused by a dominant allele, two parents with smooth chins could not have a child with a cleft chin. While there is definitely a strong genetic influence, as parents with cleft chins have a higher proportion of cleft-chin offspring than do parents without cleft chins, the 4 cleft-chinned offspring from smooth X smooth parents do not fit the myth that cleft chin is determined by a dominant allele.
Guha (1977) did a similar study on Indian families:
| Parents | Cleft offspring | Smooth offspring | Percent cleft |
|---|---|---|---|
| Cleft x Cleft | 30 | 1 | 97% |
| Cleft x Smooth | 78 | 30 | 72% |
| Smooth x Smooth | 3 | 250 | 1% |
If cleft chin was caused by a dominant allele, the Hardy-Weinberg relationship predicts that the frequency of the C allele is 15%. Most of the C x C matings would then be of two CS heterozygotes, so close to 1/4 of the offspring of two C parents should be smooth, not 3%. So like the data of Beckman et al. (1960), these data indicate that cleft chin is strongly influenced by genetics, but it's not the simple one-gene, two allele system as described in some textbooks.
Twin studies
Guha (1977) observed 53 pairs of identical twins; both twins had cleft chins in 21 pairs, and both twins had smooth chins in 32 pairs. This is evidence that cleft chin is primarily determined by genetics, not the environment, but it does not say how many genes are involved.
Other genetic evidence
Günther (1939), Beckman et al. (1960) and Guha et al. (1977) reported that about twice as many males as females had cleft chins. The large difference in frequency between the sexes does not fit the simple genetic model of cleft chin being determined by a single autosomal (non-sex-linked) gene.
Bhanu and Malhotra (1972) recorded the frequency of cleft chin in Indian boys and men of different ages. They said there was no difference in frequency of cleft chin among age groups, using a chi-square test, but when I analyze the data using logistic regression, I get a significant (P=0.03) increase in cleft chin with age; about 5% of boys 6 to 10 years old have cleft chins, while 10% of men over 35 have a cleft chin. This change with age is also evidence against the simple genetic model.
Molecular data
The consumer DNA companies Ancestry.com and 23andMe ask their customers whether they have a cleft chin. 23andMe says they've found 38 genetic markers that are related to cleft chin, while Ancestry.com says SNP rs11684042 on chromosome 2 is involved, but is not the only marker that is. As near as I can tell, neither company has published any of their results on cleft chin. It sounds like their results are consistent with the family and twin studies--cleft chin is highy affected by genetics, but by the combination of more than one gene, not a simple one-gene, two-allele system.
Further research
Maybe it would be worthwhile to measure the elevation profile across a bunch of chins, to demonstrate that there's no clear distinction between cleft and smooth chins, just gradual variation. I don't know the best way to easily and accurately measure chins; maybe something involving lasers? Other than that, I can't think of further research involving cleft chins.
References
Beckman, L., J.A. Böök, and E. Lander. 1960. An evaluation of some anthropological traits used in paternity tests. Hereditas 46: 543-569.
Bhanu, V., and K.C. Malhotra. 1972. A population genetic study of cleft chin in India. American Journal of Physical Anthropology 37: 367-372.
Günther, H. 1939. Anomalien und Anomaliekomplexe in der Gegend des ersten Schlundbogens. Zeitschrift für menschliche Vererbungs- und Konstitutionslehre 23: 43-52.
Guha, A. 1977. Twin and family studies in inheritance of cleft chin. Man in India 57: 170-177.
Lebow, M.R., and P.B. Sawin. 1941. Inheritance of human facial features: a pedigree study involving length of face, prominent ears and chin cleft. Journal of Heredity 32: 127-132.
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This page was last revised December 8, 2011. Its address is http://megalorchestia.com/mythcleftchin.html. It may be cited as pp. 23-25 in: McDonald, J.H. 2011. Myths of Human Genetics. Sparky House Publishing, Baltimore, Maryland.
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